The invention relates to the field of tape drive and tape drive heads for linear recording tapes (e.g. linear magnetic tapes). In particular, it relates to jointly optimized dimensional parameters of servo readers of the tape drive head.
Various data storage media or recording media such as magnetic tape, magnetic disks, optical tape, optical disks, holographic disks or cards, and the like are known which allow for storage and retrieval of data. In particular, in magnetic media, data are typically stored as magnetic transitions, i.e., they are magnetically recorded in the magnetic layer of the media. The data stored is usually arranged in data tracks. A typical magnetic storage medium, such as a magnetic tape, usually includes several data tracks. Data tracks may be written and read individually, or sets of data tracks may be written and read in parallel depending on the embodiment. Optical media, holographic media and other media formats may also make use of such data tracks. Transducer (read/write) heads are positioned relative to the data tracks to read/write data along the tracks. To this aim, a tape drive head must locate each data track and accurately follow its path. To achieve this, servo techniques have been developed which allow for a precise positioning of the head relative to the data tracks. One such technique makes use of servo patterns, that is, patterns of signals or recorded marks on the medium, which are tracked by the head. The servo patterns are recorded on the recording medium such as to provide a position reference for the data tracks. In other words, a servo head reads a servo pattern, which is then interpreted by a servo channel into a position error signal (PES). The latter is then used to adjust the distance of the servo head relative to the servo pattern and thereby ensure a proper positioning of the transducers with respect to the set of data tracks.
In a magnetic tape, the servo patterns are stored on dedicated tracks (called servo bands). A plurality of patterns may be defined within a servo band and a plurality of servo bands might be relied upon, the data tracks being arranged between the servo bands. A particular servo technique is based on the timing-based servo (TBS) pattern, which makes use of parallel and non-parallel marks, to which time or distance variables can be associated. The time/distance offset related to the detection of a set of servo marks is translated into a position signal. A position error signal (PES) can then be generated by subtracting a reference signal from the position signal to determine the position of a transducer relative to a data track.
In somewhat more detail, the TBS technology was developed specifically for linear tape drives in the late '90s. In a TBS system, a linear recording tape has one or more servo patterns which consist of transitions with two different azimuthal slopes. A typical linear recording tape 10 is illustrated in FIG. 1. It notably shows a servo band 20 with a typical servo pattern 21, the latter depicted in more details in FIG. 2. While reading or writing the tape, the tape moves relative to the head in the longitudinal direction x of the tape. In addition, the tape drive head 110 is moved in the direction y, i.e., transverse to the direction x. In FIG. 1, the arrow FD denotes a forward direction of the tape motion. As explained above, the position of the head is derived from the relative timing of pulses generated by a servo reader reading the pattern, which obviously depends on the exact y-position of a servo-reader of the head. FIG. 2 illustrates the geometry of a servo pattern as specified in the so-called linear tape-open (LTO) format.
For instance, assuming a constant velocity of a tape comprising the following servo pattern “//// \\\\\”, the relative timing of pulses generated by successive marks “/////” and “\\\\\” increases as the head moves downwards; it decreases as the head moves upwards. Thus, for a given pattern, a known period corresponding to a given servo location line defined in the pattern and a constant tape velocity, the distance between the servo location and the lateral position corresponding to a target data track can be monitored. TBS patterns are implemented in magnetic tape media but may also be useful in other media.
The complete format for LTO drives of generation 1 (LTO-1) was standardized by the European Computer Manufacturers Association (ECMA) in 2001 as ECMA-319. Additional information on LTO technology, in particular on LTO drives of generations 2 to 5 (LTO-2 to LTO-5), where the servo format was not modified, can be found on the World Wide Web (www), e.g., at ultrium.com. TBS patterns also allow the encoding of additional longitudinal position (LPOS) information without affecting the generation of the transversal position error signal (PES). This is obtained by shifting transitions from their nominal pattern position x as also shown in FIG. 2.
In further detail, and in reference to FIGS. 1 and 3, a servo pattern 21 is prerecorded in several servo bands distributed across the tape, e.g., five bands in the LTO servo format (and some proprietary servo formats as well). Storage data is recorded in the regions 30 (data bands) located between pairs of servo bands. A data band 30 is partitioned into m sub-bands 31-3m that correspond to the data read/write transducers hosted in the head, i.e., m is equal to the number of transducers simultaneously reading or writing m data tracks, which form a wrap. Each sub-band is partitioned into n tracks that belong to the n wraps. For example, in FIG. 2 the bold tracks 311, 321, . . . , 3m1 correspond to what is usually referred to as wrap 0 for data band 0 in the LTO specification. FIG. 1 schematically illustrates the positioning of the five servo bands and the four data bands 30 as specified in the LTO format. In the read/write heads of LTO and some Enterprise tape drives, at least two servo readers are normally available per head module, from which LPOS information as well as position information can be derived. In addition, the head of the tape drive typically consists of at least two head modules. For example, in FIG. 3 the left head module 110a of head 110 hosts at least two servo readers 111a. 
Several methods exist for the detection of the servo patterns. Such methods ensure the processing of a servo signal for the generation of not only the lateral position y-estimates but also for the generation of velocity estimates of a tape relative to tape drive head in the longitudinal x direction, which are then employed for the control of track-following and reel-to-reel servomechanisms of the tape drive. It turns out that the quality of the PES estimates, which are obtained from the y-estimates, depends on various parameters of the servo patterns. One such parameter is the azimuth angle α (α=6° for LTO standards 1 to 5). Other parameters are the servo reader width and servo stripe width s, that is, the minimum distance between magnetic transitions (e.g., s=2.1 microns (μm) for LTO 1 to 5). The width of the servo band is usually the same from one standard to another. These parameters impact the resolution of the position error signal used for track following. Ultimately, the resolution of this signal limits track following performance and hence the track density that can be achieved. At high storage areal densities, an optimization of the servo pattern geometry is useful to achieve a minimum value of standard deviation of the PES.
Efforts have been made to optimize the servo patterns. In particular, servo pattern optimization has focused on the choice of: (i) parameters defining the servo pattern geometry, e.g., the azimuth angle α, which increases the position signal resolution, and (ii) the pattern itself, e.g. for minimizing written-in velocity errors. For example, it is known that increasing the azimuth angle from 6 to 12 or even 18 degrees, a substantial improvement in the quality of position signal estimation can be achieved, which translates into lower values of PES standard deviation. Similarly, it is known that the so-called “M” and “N” servo patterns lead to PES estimation, which is insensitive to written-in velocity errors.
Next, in linear tape drives, the m read/write transducers are evenly spaced at, e.g., a pitch of 166.5 μm across the width of a data band of 2664 μm in LTO 4. The data band area is written/read by writing/reading m tracks simultaneously forming a wrap and laying out the wraps in a serpentine fashion. This is reflected in FIG. 3, see the successive arrows “As” indicating tracks that belong to different wraps. A data band 30 is for instance filled by running the tape forth and back n times forming 2n wraps, shifting the position of the read/write transducers to another wrap location at each pass and this operation is repeated, until the entire data band is filled. The position in the servo band 20 that corresponds to a given wrap (e.g., the tracks 311, 321, . . . , 3m1 of wrap 0) is referred to as a servo location. Two such servo locations are depicted by horizontal gray lines 25 in FIG. 3. As explained above, the servo pattern must provide position information for positioning the head at each of these servo locations. Hence, the width of the servo band (186 μm in the example of FIG. 1) is primarily determined by the pitch between adjacent transducers in the read/write head, in the direction transverse to the tape direction. This pitch is in turn determined by the number m of parallel channels implemented. For example, LTO 4 drives comprise 16 parallel data channels, resulting in a pitch between writers of ˜2859/16˜166.5 μm. This substantially corresponds to the typical servo band width, i.e., 186 μm, subject to a margin that accounts for the width of the servo reader and for tracking errors. Accordingly, the servo band width is chosen such as to provide tracking information for all wrap locations in a data band.
In addition to the resolution of the y-position estimates described above, it can be realized that another parameter is important for determining the performance of the track following control system, which is the update rate of the position estimates. A high update rate is of particular importance for operation at low tape velocities, which is required for matching the drive data rate to the data rate of slower hosts. Thus, ideally, one may want to increase both the resolution of the pattern as well as the update rate of the lateral position and velocity estimates.
As the tape drive systems for linear tape formats such as the LTO format typically have one or two heads, each head having an array of transducers for writing to and reading from the tape. For example, a state-of-the-art multichannel tape magnetic recording head today contains sixteen data channels and two servo reader channels in each of two bidirectional modules. Each servo reader typically comprises a sensor (for sensing a magnetic field component when reading the servo pattern), the sensor located between two shields, one on each side of the sensor. The shields are configured to shield spurious components of magnetic fields occurring upon reading the tape. Typically, the shield-to-shield gap (i.e., spacing) in a servo reader is set and scaled based on the design of the data readers. For instance, a usual practice is to fabricate the servo reader channels using the same shield-to-shield gap dimensions as those in the data reader channels, as this minimizes fabrication costs. Finally, as technology advances, the data reader gaps are optimized to thinner dimensions, providing for detection of higher linear densities of magnetic transitions along the tape. To summarize years of evolution in this field, servo reader shield-to-shield gaps evolved from around 0.35 μm for the first generation of LTO down to 0.18 to 0.3 μm for current tape drives.
Other dimensions of the servo readers like the servo reader width (perpendicular to the gap and hence perpendicular to the longitudinal direction of motion of the tape during normal drive operation) usually attract less attention, if not at all. For instance, the servo reader width has been almost systematically set to 6 to 8 μm.
U.S. Pat. No. 7,760,465 discloses mechanisms for optimizing multiple read channels of different varieties on a magnetic recording head uniquely for performance, reliability, and/or thermal characteristics, while still building (fabricating) the multiple readers simultaneously. Embodiments of this invention provide increased signal amplitude and increased protection against shorting in some channels contained in advanced multichannel narrow gap recording heads, where the gap of these channels do not require the narrowest gap of the population of channels in that head. The magnetic heads have multiple reader channels deposited simultaneously as shielded magnetoresistive transducers (e.g., GMR devices, Anisotropic Magnetoresistive (AMR) devices, Tunneling Magnetoresistive (TMR) devices, etc.). The deposited nonmagnetic gaps to each shield (upper and lower) of each transducer (channel) may be of different thicknesses in order to optimize that channel's characteristics. Varying gap size is regarded as important for performance characteristics, such as for reading a particular linear density. A smaller gap is desirable for reading a tape with a high linear data density because the resolution of the reader is finer. However, a reader with a larger gap provides a stronger signal (higher amplitude) and a higher Signal to Noise (S/N) ratio as compared to a reader with a smaller gap when reading lower linear densities. A higher amplitude is regarded desirable for such functions as reading the servo track on the tape. However, as mentioned above, increasing the gap reduces the signal resolution, which eventually offsets the benefits of an increased S/N ratio, even for low density patterns. For example, the servo pattern used on tapes may be the same from format family to format family, e.g., LTO-1 to LTO-2 may use the same servo pattern, though the linear data density of the data tracks may have increased. Accordingly, the gaps of the servo readers are thicker than the gaps of the data readers (in some embodiments), providing increased signal amplitude and increased reliability (e.g., protection from shorting) in the servo readers, while having the high data resolution provided by the narrow gaps of the data readers.